I doubt it will be fully legal by 2018. At the state level, I think it's likely more states will decriminalize or even fully legalize, but not all of them. I'll put a guess at: by 2018, it will be fully legal in 15 states, decriminalized in 15, and still criminal to possess in 20 states.
At the federal level, the Single Convention on Narcotic Drugs would complicate legalization efforts, since the Treaty requires signatories to ban marijuana. And the U.S. doesn't want to undermine this treaty, because it uses the treaty to strongarm other countries over things like coca and poppy growing. It's possible the DEA will lower enforcement priority, though, and maybe possible (though imo this is less likely) that Congress will revise the law to reduce sentences from their current levels.